1. Field of the Invention
The subject invention relates generally to a vehicular wheel hub assembly including an integrated oil seal and speed sensor feature.
2. Related Art
Vehicle speed sensors integrated into a wheel assembly have many useful purposes. One common use for a speed sensor assembly is to cooperate with the vehicle anti-lock braking system. During a vehicle braking condition, a speed signal generated by pulsed electrical signals from the speed sensor assembly is sent to an on-board computer that responds to the drop in vehicle speed. If a wheel lock-up condition is anticipated, the computer directs a valve in the brake system to relieve or modulate the fluid pressure within the brake assemblies at the wheel thereby preventing an undesired wheel lock-up condition. When the computer determines that a lock-up condition is no longer imminent, the braking pressure is returned to normal operation.
In the case of larger trucks and commercial vehicles, it is common to incorporate a so-called oil bath seal between the rotating hub and its stationary spindle to keep lubricating fluid contained within the confines of the roller bearing assemblies. For such applications in which a vehicle speed sensor is desired, it has been taught to incorporate the target portion of the speed sensor assembly into the oil bath seal. In other words, the target for the speed sensor assembly is mounted to that part of the oil bath seal which rotates with the hub about the stationary spindle. The stationary portion of the sensing device, i.e., the sensor per se, is directed at the target in close proximity so that electrical pulses can be generated as the hub rotates, which electrical pulses are then converted into vehicle speed.
According to prior art techniques, speed sensor assemblies such as used in vehicle wheel systems fall into two general categories, namely variable reluctance and inductive type sensor systems. A variable reluctance sensor is a passive device which projects a magnetic field through a coil toward a ferrous target which serves as an actuator. As the actuator target moves, its discontinuities in the form of gear teeth, blades, etc., excite a voltage in the coil, thus producing an electrical sinusoidal wave current with frequency and voltage proportional to the target velocity. As each discontinuity passes by the pickoff coil, it generates a pulse and a pulse train as cycles are repeated. Variable reluctance type sensors are sometimes preferred for the comparatively large voltage amplitudes generated in operation.
Inductive sensors, while somewhat similar in configuration to the variable reluctance type and which generate the same type of signal, are nevertheless distinguished in that its inductive pickoff coils have no internal permanent magnet. Rather, an inductive sensor relies on external magnetic field fluxuations, such as a rotating permanent magnet in order to generate signal pulses. The rotating permanent magnet is often referred to as an encoder ring, and has never been used with the older-style VR sensor because the magnetic flux field produced by the encoder ring has been thought too weak to produce sufficient signal strength.
While both types of sensor assemblies have been proposed for use in vehicle wheel sensing applications, the large truck or commercial vehicle applications in which an oil bath seal is used in connection with a rotating hub and spindle assembly more typically make use of the variable reluctance (VR) type sensor configuration. The target for a VR system, which is also referred to as a tone ring, may be integrated with an oil bath seal. The tone ring is characterized by its thick, gear-like teeth or otherwise crenulated ring-like features. One example of a variable reluctance sensor for use in this type of application may be found in U.S. Pat. No. 5,476,272 to Hixon, grated Dec. 19, 1995 and assigned to the Assignee of this invention.
Because of the fundamental differences between the variable reluctance-type sensors and the inductive-type sensors, during maintenance operations when an oil bath seal is removed, it is required that its replacement include a tone ring style target for the variable reluctance sensor having the same number of teeth or crenulations as the part it is replacing. And while the tone ring style target designs have become somewhat disfavored because of their comparatively lower durability, thickness/weight and susceptibility to debris accumulation and corrosion, changing to an inductive type sensor would require the added expense and labor of also changing its inductive pickoff coil to the type used in an inductive-type sensor assembly. Accordingly, once a variable reluctance type sensor assembly is installed in a vehicle, subsequent maintenance operations continue to require an older style disfavored tone ring type target even though industry preferences are moving toward an inductive type sensor assembly.